: She is saying that this person has not really applied himself
(or herself) to any serious job or vocation, and has gone through
life with no real personal or professional aim in mind. The response
suggests that he (or she) may not be the only one in the family
with this outlook on life.

The speaker is using "knock about" in the sense described by this
definition in the Oxford English Dictionary: "to move about, wander,
or roam, in an irregular way; also to lead an irregular life" (colloquial).
The character who is 29 admits to having followed an erratic course.
"Rolling stone" is a similar expression. A rolling stone travels
without direction from within and knocks into things as it goes.